They are fairly close synonyms. They are fairly interchangeable. They are at about the same "register" or level of formality.
To me, "fate" often focusses on supposed supernatural forces that steer events, and "destiny" often refers to the final outcome. Fate steers you to your destiny.
"Destiny" often means one specific outcome. It is beyond our control, it can't be changed, and it is certain to happen even if you don't know exactly how it is going to happen. For example, "all soap bubbles are destined to burst."
"Fate" often refers to the supernatural forces that steer events. However, the word can <em>also</em> used to refer to the final outcome.